The subject invention relates to a television monitor capable of displaying wide aspect ratio pictures having an aspect ratio of, for example 9:16 and, more particularly, capable of selectively displaying standard width pictures having an aspect ratio of 3:4.
A television monitor is characteristically produced with a layer of luminous phosphor on the inside surface of its face plate. The phosphor is selectively impinged by an electron beam and, as a result, luminesces forming the picture to be displayed. Over the course of time, due to this exposure to electron beams, the phosphor loses a certain amount of efficiency, this irreversible degradation being known as "Coulomb aging". Although the loss of phosphor efficiency varies in different ways according to the beam current time of exposure, and other details, in the following, "Coulomb aging" will be used to refer to the phenomenon of loss of phosphor efficiency as a consequence of exposure to a raster scan.
Applicants have determined that a differential scanning of a phosphor screen in a CRT of as few as one Coulomb will produce a 1 percent difference in phosphor luminance efficiency. An abrupt step change of 1 percent luminance is visible to the casual observer.
Hence, in implementing a dual aspect ratio monitor, it is necessary to manage the effect of differential loss of phosphor efficiency. This differential aging is a direct consequence of using less of the phosphor screen width when displaying a 3:4 aspect ratio picture on a 9:16 aspect ratio display in which the height has been kept constant. In particular, that portion of the phosphor required to display the 3:4 aspect ratio picture receives an average additional amount of charge proportional to the average instantaneous beam current times the time that this standard picture is being displayed. During the portion of time that a 9:16 aspect ratio picture is being displayed, all of the phosphor, on the average, receives the same charge per unit area and no problem arises.
The percentage of phosphor used for displaying 9:16 aspect ratio pictures but not used to display 3:4 aspect ratio pictures is readily calculated. If the height of the phosphor used is h, then in the first case, the width used is h.times.4/3, and in the second case the width used is h.times.16/9. Thus, the fraction of width unused in the first case is: EQU (h.times.16/9-h.times.4/3)/(h.times.16/9)
The h term cancels out. Multiplying the numerator and denominator by 9 to eliminate fractions yields: EQU (16-12)/16 or 4/16 or 1/4.
Thus, the standard aspect ratio picture has 25 percent less picture width for the same picture height than the wide aspect ratio picture (see FIG. 1). Conversely, the wide aspect ratio picture has 331/3 percent greater width.